And here's what Metropolitan Police commissioner Bernard Hogan-Howe writes in paragraph 13 of his report to next week's meeting of the full Metropolitan Police Authority, comparing April to September 2011 with the same stretch of time last year:

The overall number of victims of Serious Youth Violence increased by 9.5% (328 more offences from 3,435 to 3,763) over the 6 month period.

Boris's false claims and figure-fiddling over crime are fast attaining elephantine scale. In February, he associated himself with falls in public transport-related offences that happened six months before he'd made any changes to the relevant policing arrangements and four months before he'd even become mayor.

His efforts to divert attention from having been caught out repeatedly misrepresenting the achievements of his youth offender re-settlement project included calling the distinguished chair of the independent UK Statistics Authority "a Labour stooge." Adam Bienkov has revealed that his campaign website is a treasure trove of crime stat spin and plain inaccuracy.

Let's try to get an honest handle on all this. As I wrote last week at Comment is Free, there is a large, good news story about crime in London and a smaller, very bad one.

The good news story is that overall reported crime of all kinds has been in general decline in the capital for years, a trend that far pre-dates the arrival of Boris at City Hall. The bad news story is that serious violent crimes against children and young adults are following a different trend from that for crime in general.

While all youth violence - offences against under 20s - came down slightly during the first three years of Boris's term, the offences categoried by the Met as "serious" crept up - and as Bernard Hogan-Howe's report reveals, that has continued.

Furthermore, there was a very large jump in the number of 13-24 year-olds injured with knives in the second and third years of Boris's term compared with the first, despite the Met's increased use of the stop-and-search tactic and the introduction of knife-detecting arches at transport hubs.

The commissioner's report also confirms that "London has experienced an increase in knife crime offences over the last three years which has continued in 2011," adding that "offences are now almost one fifth higher than 2009 levels."

Boris came to power promising to vigorously tackle serious youth violence and knife crime. As he seeks re-election he is claiming to have achieved a great deal. As his police commissioner has now helpfully testified, these claims seem not to be based in reality.

Update, 16:45 Two people have been in touch. One, a Labour supporter, drew my attention to candidate Boris's criticism of then mayor Ken Livingstone's attitude to fatal stabbings of teenagers, as quoted in the Evening Boris newspaper during the 2008 campaign:

This is a Mayor who is clearly out of touch. He either has no idea what is going on in his own city or he is lying.

The other person to get in touch was one of the mayor's press officers to deliver the following statement:

When responding to James Cleverly's MQT question the Mayor was referring to the latest MPA statistics on youth violence which show a decrease of 8.6 per cent from April 2011 to October 2011

In other words, Boris's office says he just made a mistake: although he said "serious youth violence," he meant all youth violence, not just those categorised by the Met as "serious". As I've said above, the stats for all youth violence - meaning violent offences against victims victims under the age of 20 - have fallen during his mayoralty, but those for the serious types have gone up.